There are many circuit applications in which there is a useful signal present and the information which that particular signal represents must be provided to a significantly higher load than the signal itself can provide. This situation can arise in many different situations. The large load may be a highly capacitive line such as a bus line in a microprocessor or a large number of circuits such as decoders in a memory which must be clocked by the signal. The signal may exist at the desired voltage level but simply not have the drive capability needed. This general type of problem is addressed by a type of circuit which at least includes current amplification. These circuits are sometimes called line drivers or current drivers.
In the typical problem in which the particular current driver is present in an integrated circuit, the load typically includes high capacitance. The typical solution is to have a bipolar transistor, if one is available, to operate in an emitter-follower mode to provide the actual output to the large load. Operating in the emitter-follower mode, the bipolar transistor provides only current gain and no voltage gain. There is a reduction of the bias voltage from the base to the emitter of one base-emitter voltage drop (Vbe) which must also be taken into account. The emitter-follower is particularly effective for logic low to logic high transitions because the bipolar transistor actively provides current to the particular load. The logic high to logic low transition, however, can be a problem if the load is highly capacitive. The problem is that for the logic high to logic low transition the bipolar transistor becomes non-conductive and there is then nothing which actively reduces the voltage on the load. If the load is mostly resistive, then the logic high to logic low transition is very fast, but if it is highly capacitive, the discharge of the capacitance can be comparatively slow.
The typical solution to this problem of discharging a highly capacitive load is to provide a current source connected to the emitter of the bipolar transistor to discharge the capacitance. This has been shown to be very effective but disadvantageously increases the current required to achieve the requisite speed for the logic high logic low transition. A relatively large current for the current source improves speed for the logic high to logic low transition but not only provides the disadvantage of using more current but also provides for a decrease in speed for the logic low to logic high transition. There is thus a trade-off between improving the speed of the logic high to logic low transition while detrimentally effecting current consumption and speed of the logic low to logic high transition.